1. Field of the Invention
The invention relates to mobile networks, and more particularly to positioning a user equipment accessing multiple mobile networks.
2. Description of the Related Art
User equipments, such as a notebook or a cell phone, are wirelessly connected to a mobile network for communication. When user equipment is connected to a mobile network, the mobile network requires location information of the user equipment, to provide the user equipment with a communication link with better communication quality. Additionally, a user equipment also requires its own location information, to adjust its position to obtain a wireless link with better communication quality. A method for positioning user equipment connected to a mobile network is therefore required.
Conventional methods for positioning a user equipment comprises a cell-identity method, an observed time difference of arrival (OTDOA) method, an uplink-time difference of arrival (UTDOA) method, and a network-assisted GPS positioning (A-GPS) method. Referring to FIG. 1A, a schematic diagram of a cell-identity method for positioning a user equipment 102 is shown. Assume a mobile network comprises a base station 104 and a radio network controller (RNC) 106. When the user equipment 102 is connected to the mobile network, the mobile network sends a request for locating the user equipment 102 to the radio network controller 106. The radio network controller 106 then sends a state transition signal to the user equipment 102 via a base station 104 connected to the user equipment 102. After the base station 104 responds to the state transition signal with a state transition success signal, the radio network controller 106 can identify the base station 104 connected to the user equipment 102, and determine the location of the user equipment 102 according to a service coverage area of the base station 104.
Referring to FIG. 1B, a schematic diagram of an observed time difference of arrival (OTDOA) method for positioning a user equipment 130 is shown. Assume that a mobile network comprises three base stations 122, 124, and 126 located in the vicinity of the user equipment 130. The base stations 122, 124, and 126 broadcast a signal and the user equipment 130 derives its location according to the signals broadcasted by the three base stations. The variable distances between the user equipment 130 and the base stations 122, 124, and 126 cause the signals broadcasted by the base stations 122, 124, and 126 having different signal delays. A first phase difference exists between the signals broadcasted by the base stations 122 and 126, and a second phase difference exists between the signals broadcasted by the base stations 124 and 126.
A hyperboloid 132 corresponding to probable locations of the user equipment 130 is derived according to the first phase difference and the locations of the base stations 122 and 126, and a hyperboloid 134 corresponding to probable locations of the user equipment 130 is derived according to the second phase difference and the locations of the base stations 124 and 126. The two hyperboloids 132 and 134 are drawn with widths to show the errors induced by calculating the probable locations fom the phase difference and the base station locations. A cross point of the hyperboloids 132 and 134 indicates a most probable position of the user equipment 130. The OTDOA method shown in FIG. 1B estimates a position of user equipment according to phase difference between signals emitted by the base stations of a mobile network. Similarly, an uplink-time difference of arrival (UTDOA) method estimates a position of a user equipment according to phase difference between signals received by base stations of a mobile network based on similar algorithms.
Referring to FIG. 1C, a schematic diagram of a network-assisted GPS positioning (A-GPS) method for positioning a user equipment 152 is shown. A mobile network comprises a serving radio network controller (SRNC) 154 and a Stand-Alone Serving-mobile-location-center (SAS) 156. The user equipment 152 comprises a global positioning system (GPS) module. The mobile network first sends a request to the SRNC 154 for positioning the user equipment 152. The SRNC 154 then collects network information about the user equipment 152 and sends an SAS invoke signal comprising the network information about the user equipment 152 to the SAS 156. The SAS 156 then delivers A-GPS assistance information to the user equipment 152 in response to the SAS invoke signal. The user equipment 152 then performs GPS measurement with the aid of A-GPS assistance information and sends the GPS measurement to the SAS 156 through the SRNC 154. The SAS 156 then calculates location information of the user equipment 152 according to the GPS measurement and then provides the SRNC 154 with location information of the user equipment 152. Finally, the SRNC 154 forwards the location information to the mobile network.
The conventional cell-identity method, OTDOA method, UTDOA method, and A-GPS method provides location information of a user equipment with limited precision. User equipment often can access more than one mobile network and obtain location information from more than one mobile network. Referring to FIG. 2, a schematic diagram of user equipment 202 capable of accessing three mobile networks 212, 214, and 216 is shown. The user equipment 202 comprises three subscriber identity modules (SIM) 222, 224, and 226. After the user equipment 202 is connected to one or more of the mobile networks 212, 214, and 216, the user equipment 202 may obtain more than one set of location information (e.g. L1, L2, and L3) from the mobile networks 212, 214, and 216. The user equipment 202, however, only relies on one set of the location information L1, L2, or L3, which does not enjoy the benefit of accessing multiple mobile networks.